Far-infrared Herschel SPIRE spectroscopy of lensed starbursts reveals physical conditions of ionised gas

Abstract

The most intensively star-forming galaxies are extremely luminous at far-infrared (FIR) wavelengths, highly obscured at optical and ultraviolet wavelengths, and lie at $z\ge 1-3$. We present a programme of $Herschel$ FIR spectroscopic observations with the SPIRE FTS and photometric observations with PACS, both on board $Herschel$, towards a sample of 45 gravitationally lensed, dusty starbursts across $z\sim 1-3.6$. In total, we detected 27 individual lines down to 3-$\ddot{I}ƒ$, including nine $[\mathrm{C}\mathrm{II}]$ 158-$\hat{I}¼$m lines with confirmed spectroscopic redshifts, five possible $[\mathrm{C}\mathrm{II}]$ lines consistent with their far-infrared photometric redshifts, and in some individual sources a few $[\mathrm{O}\mathrm{III}]$ 88-$\hat{I}¼$m, $[\mathrm{O}\mathrm{III}]$ 52-$\hat{I}¼$m, $[\mathrm{O}\mathrm{I}]$ 145-$\hat{I}¼$m, $[\mathrm{O}\mathrm{I}]$ 63-$\hat{I}¼$m, $[\mathrm{N}\mathrm{II}]$ 122-$\hat{I}¼$m, and OH 119-$\hat{I}¼$m (in absorption) lines. To derive the typical physical properties of the gas in the sample, we stack all spectra weighted by their intrinsic luminosity and by their 500-$\hat{I}¼$m flux densities, with the spectra scaled to a common redshift. In the stacked spectra, we detect emission lines of $[\mathrm{C}\mathrm{II}]$ 158-$\hat{I}¼$m, $[\mathrm{N}\mathrm{II}]$ 122-$\hat{I}¼$m, $[\mathrm{O}\mathrm{III}]$ 88-$\hat{I}¼$m, $[\mathrm{O}\mathrm{III}]$ 52-$\hat{I}¼$m, $[\mathrm{O}\mathrm{I}]$ 63-$\hat{I}¼$m, and the absorption doublet of OH at 119-$\hat{I}¼$m, at high fidelity. We find that the average electron densities traced by the $[\mathrm{N}\mathrm{II}]$ and $[\mathrm{O}\mathrm{III}]$ lines are higher than the average values in local star-forming galaxies and ULIRGs, using the same tracers. From the $[\mathrm{N}\mathrm{II}]/[\mathrm{C}\mathrm{II}]$ and $[\mathrm{O}\mathrm{I}]/[\mathrm{C}\mathrm{II}]$ ratios, we find that the $[\mathrm{C}\mathrm{II}]$ emission is likely dominated by the photo-dominated regions (PDR), instead of by ionised gas or large-scale shocks.

39 pages, 19 figures, Accepted for publication in MNRAS. For extra pptx slides prepared for this work, please see http://www.eso.org/~zzhang/download/FTS_SMG_survey_ZhiyuZhang.pdf